Low tumor blood flow and resultant tumor hypoxia are recognized as important mechanisms of resistance to anticancer treatment such as radiotherapy or chemotherapy. Therefore, drugs that can nearly totally reduce new blood vessel formation and thus limit blood flow are receiving considerable interest of therapeutics. Thus a non-invasive and quantitative method is needed in order to investigate the pretreatment and post-treatment status of tumor blood flow. However, there is no well-established clinical measurement for this purpose. Positron emission tomography (PET) is a noninvasive method to study tissue physiology and metabolism in vivo. The measurement of [oxygen-15]-water (O-15-water) intravenous-injection with PET has shown accuracy in analyzing regional blood flow of brain or myocardium. It has been shown that measurement of O-15-water diffusion with PET in tumors is feasible in a measurement of tumor blood flow. While the O-15-water method has been validated in microsphere studies in animal models, it has not been assessed for precision in human tumor in vivo. Much current interest exists in treating cancer with agents that may reduce tumor blood flow through reducing angiogenesis. In addition, tumor blood flow heterogeneity is known to be related to poor treatment response in malignant tumors. Despite the importance of tumor blood flow, there has been little systematic study of the reproducibility of minimally invasive tumor blood flow measurement studies using positron emission tomography (PET) in untreated cancer patients. The aim of the present study is: To evaluate the reproducibility of quantitative tumor blood flow measurement using O-15 water method in untreated lung cancers and other large tumors.